Engine output control system for vehicle

ABSTRACT

An engine output control system for a vehicle detects a degree of slip of driving wheels of the vehicle and determines engine output reduction requirement by which the output of the engine is to be reduced in order to converge the degree of the slip of the driving wheels on a target value according to the degree of slip detected. The output of the engine is reduced by the engine output reduction requirement when the engine speed is not lower than a preset engine speed and reduction of the engine output is inhibited when the engine speed is lower than the preset engine speed. The preset engine speed is changed according to the engine output reduction requirement so that the preset engine speed is higher when the engine output reduction requirement is large than when it is small.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an engine output control system for a vehicle.

2. Description of the Prior Art

In a so-called traction control system, the engine output is reducedindependent from the amount of depression of the accelerator pedal of avehicle when the slip of the driving wheels of the vehicle reaches apredetermined value so that the slip of the driving wheels converges ona target value. However when the engine output is reduced at a lowengine speed, the engine can stall.

In order to overcome such a problem, in a traction control systemdisclosed in Japanese Unexamined Patent Publication No. 60(1985)-121129,reduction of the engine output is inhibited when the engine speed islower than a preset engine speed (e.g., 600 to 2000 rpm). That is, inthe traction control system, slip of the driving wheels is detected onthe basis of the difference between the wheel speed of the drivingwheels and that of the driven wheels, and when the slip of the drivingwheels thus detected reaches a predetermined value, fuel supply to oneor more of the cylinders of the engine is cut to reduce the engineoutput so that the slip of the driving wheels converges on a targetvalue. When the engine speed is lower than a preset engine speed, fuelcut is inhibited.

Though the traction control system is advantageous in that the engine isprevented from stalling since fuel cut is inhibited when the enginespeed is low, it gives rise to the following problems due to the factthat the preset engine speed is fixed.

That is, when the engine output reduction requirement is large, forinstance, to such an extent that fuel supply to all the cylinders is tobe cut, the engine can stall since the preset engine speed is relativelylow for the engine output reduction requirement and the engine output isabruptly lowered. On the other hand when the engine output reductionrequirement is small, for instance, to such an extent that fuel supplyto only one cylinder is to be cut every other intake stroke, the engineoutput cannot be sufficiently reduced though the engine is not apt tostall in such conditions since the preset engine speed is relativelyhigh for the engine output reduction requirement.

SUMMARY OF THE INVENTION

In view of the foregoing observations and description, the primaryobject of the present invention is to provide an engine output controlsystem for a vehicle in which the engine output can be satisfactorilyreduced without fear of engine stall.

In accordance with the present invention, there is provided an engineoutput control system for a vehicle comprising a slip detecting meanswhich detects a degree of slip of driving wheels of the vehicle, anengine output reduction determining means which determines engine outputreduction requirement, by which the output of the engine is to bereduced, according to the degree of slip of the driving wheels detectedby the slip detecting means, an engine output reducing means whichreduces the output of the engine by the engine output reductionrequirement determined by the engine output reduction determining means,an engine speed detecting means which detects the engine speed, and anengine output reduction inhibiting or limiting means which inhibits orlimits reduction of the engine output when the engine speed is lowerthan a preset engine speed, preset engine speed changing means whichchanges the preset engine speed according to the engine output reductionrequirement so that the preset engine speed is higher when the engineoutput reduction requirement is large than when it is small.

The engine output reducing means may be in various forms. For example,it may be a means for controlling fuel supply to the cylinders of theengine, a means for controlling the ignition timing or a means forcontrolling the amount of intake air. The engine output reductioninhibiting means may inhibit reduction of the engine output or may limitreduction of the engine output to a fraction (e.g., a half) of theengine output reduction requirement when the engine speed is lower thanthe preset engine speed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a vehicle provided with an engineoutput control system in accordance with an embodiment of the presentinvention,

FIG. 2 is a view showing a map for determining the reference enginespeed,

FIG. 3 is a flow chart showing the operation of the CPU in control ofengine output,

FIG. 4 is a view showing another map for determining the referenceengine speed, and

FIG. 5 is a flow chart showing a modification of the operation of theCPU in control of engine output.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1, an engine 9 has six cylinders 10, number one cylinder tonumber 6 cylinder, though only one cylinder is seen therein. The engine9 is provided with an airflow sensor 1, throttle body 2, a surge tank 5and an intake manifold 7 which communicates an intake port 6 for eachcylinder 10 with the surge tank 5. Fuel injectors 8 for each cylinder 10are provided in the intake manifold 7. The throttle body 2 has athrottle chamber 3 therein and a throttle valve 4 is provided in thethrottle chamber 3. An intake valve 12 is provided in each intake port6. An exhaust port 11 for each cylinder 10 is communicated with adownstream side exhaust passage 18 by way of an exhaust manifold 16. Anexhaust valve 13 is provided in each exhaust port 11, an O₂ sensor 17 isprovided in the exhaust manifold 16, and a catalytic convertor 19 isprovided in the downstream side exhaust passage 18.

Reference numeral 15 denotes a spark plug, reference numeral 20 denotesan intake air temperature sensor, reference numeral 21 denotes an idlesensor, reference numeral 22 denotes a throttle sensor, referencenumeral 23 denotes an engine coolant sensor, reference numeral 26denotes a knock sensor, reference numeral 27 denotes an ignitor coil,and reference numeral 28 denotes a distributor.

The engine 9 is mounted on a front-engine front-drive vehicle 29 whichhas left and right front wheels (driving wheels) 30 and 31 and left andright rear wheels 32 and 33 (driven wheels). The wheel speeds of therespective wheels 30 to 33 are detected by wheel speed sensors 34 to 37.

A CPU (central processor unit) 40 receives signals representing thewheel speeds of the respective wheels 30 to 33 from the wheel speedsensors 34 to 37, a signal representing the amount of intake air fromthe airflow sensor 1, an idle signal from the idle switch 21, a signalrepresenting the throttle opening from the throttle sensor 22, and anengine speed signal and a crank angle signal from the distributor 28.The CPU 40 controls the ignitor coil 27, the spark plugs 15 and the fuelinjectors 8 on the basis of those signals according to a program storedin a RAM 39. The RAM 39 stores required data such as maps shown in FIG.2 and the following table. The map shown in FIG. 2 is for determiningthe aforesaid preset engine speed below which the engine outputreduction inhibiting means should inhibit reduction of the engineoutput, and the map shown in the following table is for determining themanner of fuel cut for a given engine output reduction requirement.

                  TABLE                                                           ______________________________________                                        reduction                                                                     level    #1    #2        #3  #4      #5  #6                                   ______________________________________                                        1        x                                                                    2              xx                                                             3              xx        x                                                    4        xx    xx                                                             5        xx    xx        x                                                    6        xx    xx            xx                                               7        xx    xx            xx      x                                        8        xx    xx        xx  xx                                               9        xx    xx        xx  xx          x                                    10       xx    xx        xx  xx      xx                                       11       xx    xx        xx  xx      xx  x                                    12       xx    xx        xx  xx      xx  xx                                   ______________________________________                                    

In the above table, "x" indicates that fuel supply to the cylinder is tobe cut every other intake stroke, and "xx" indicates that fuel supply tothe cylinder is to be cut every intake stroke. For example, for theengine output reduction requirement level (abbreviated as "reductionlevel" in the table) 3, fuel supply to No. 2 cylinder is cut everyintake stroke and that to No. 3 cylinder is cut every other intakestroke.

The operation of the CPU 40 in control of engine output will bedescribed with reference to the flow chart shown in FIG. 3, hereinbelow.

In step S1, the CPU 40 reads the engine speed Ne, and the wheel speedsLFs, RFs, LRs and RRs of the left front wheel 30, the right front wheel31, the left rear wheel 32 and the right rear wheel 33. Then the CPU 40calculates in step S2 the degree of slip SF of the driving wheels, i.e.,the left and right front wheels 30 and 31, relative to the road surfaceon the basis of the wheel speeds of the driving wheels and the drivenwheels. Though the degree of slip SF may be calculated according tovarious known formulae, the degree of slip of the driving wheels SF iscalculated on the basis of formula Ds-(LRs+RRs)/2 in this particularembodiment, wherein Ds represents the larger of the wheel speeds LFs andLRs of the left and right front wheels 30 and 31. Then in step S3, theCPU 40 calculates the engine output reduction requirement by which theoutput of the engine is to be reduced in order to converge the degree ofslip of the driving wheels SF on a target value according to the degreeof slip SF of the driving wheels calculated in step S2 and determinesthe level of the engine output reduction requirement (level 1 to level12). Thereafter the CPU 40 sets in step S4 the manner of fuel cut FCwith reference to the map shown in the above table. IN step S5, the CPU40 sets a reference engine speed on the basis of the level of the engineoutput reduction requirement with reference to the map shown in FIG. 2.As can be seen from FIG. 2, the reference engine speed Nd is setstepwise according to the level of the engine output reductionrequirement so that it is the lowest for level 1 and level 2 and thehighest for level 7 to level 12. When the actual engine speed Ne islower than the reference engine speed Nd, the CPU 40 causes the fuelinjectors 8 to perform the normal fuel injection, and when the former isnot lower than the latter, the CPU 40 inhibits one or more of the fuelinjectors from injecting fuel according to the manner of fuel cut FC setin step S4. (steps S6 to S8) That is, in this particular embodiment,reduction of the engine output is inhibited irrespective of the degreeof slip SF of the driving wheels when the actual engine speed Ne islower than the reference engine speed Nd.

As can be understood from the description above, the reference enginespeed is set high when the engine output reduction requirement level ishigh in the embodiment described above, and is set low when the engineoutput reduction requirement level is low. Accordingly, the engine isprevented from stalling when the engine output reduction requirementlevel is high while the engine output can be sufficiently reduced whenthe engine output reduction requirement level is low.

Though, in the embodiment described above, the reference engine speed Ndis changed stepwise according to the level of the engine outputreduction requirement, the reference engine speed Nd may be changedlinearly with change in the level of the engine output reductionrequirement as shown in FIG. 4. Further, though, in the embodimentdescribed above, reduction of the engine output is inhibitedirrespective of the degree of silp SF of the driving wheels when theactual engine speed Ne is lower than the reference engine speed Nd,reduction of the engine output may be limited to a fraction, e.g., 2/1,of the engine output reduction requirement as shown by step S7' in theflow chart shown in FIG. 5.

What is claimed is:
 1. An engine output control system for a vehiclecomprising a slip detecting means which detects a degree of slip drivingwheels of the vehicle, an engine output reduction determining meanswhich determines engine output reduction requirement by which the outputof the engine is to be reduced in order to converge the degree of slipof the driving wheels on a target value according to the degree of slipof the driving wheels detected by the slip detecting means so that theengine output reduction requirement is larger when the slip is largethan when the slip is small, an engine output reducing means whichreduces the output of the engine by the engine output reductionrequirement determined by the engine output reduction determining meansso that the output of the engine is more reduced when the engine outputreduction requirement is large than when the engine output reductionrequirement is small, an engine speed detecting means which detects theengine speed, an engine output reduction inhibiting means which inhibitsreduction of the engine output when the engine speed is lower than apreset engine speed, a preset engine speed changing means which changesthe preset engine speed according to the engine output reductionrequirement so that the preset engine speed is higher when the engineoutput reduction requirement is large than when the engine outputreduction requirement is small.
 2. An engine output control system asdefined in claim 1 in which said preset engine speed is set stepwiseaccording to the level of the engine output reduction requirement.
 3. Anengine output control system as defined in claim 1 in which said presetengine speed is linearly increased with increase in the engine outputreduction requirement.
 4. An engine output control system as defined inclaim 1 in which said engine output reduction inhibiting means inhibitsreduction of engine output to a fraction of the engine output reductionrequirement when the engine speed is lower than the preset engine speed.5. An engine output control system as defined in claim 1 in which saidengine output reducing means reduces the output of the engine by cuttingfuel supply to one or more of the cylinders of the engine.
 6. An engineoutput control system for a vehicle comprising a slip detecting meanswhich detects a degree of slip of driving wheels of the vehicle, anengine output reduction determining means which determines engine outputreduction requirement by which the output of the engine is to be reducedin order to converge the degree of slip of the driving wheels on atarget value according to the degree of slip of the driving wheelsdetected by the slip detecting means so that the engine output reductionrequirement is larger when the slip is large that when the slip issmall, an engine output reducing means which reduces the output of theengine by the engine output reduction requirement determined by theengine output reduction determining means so that the output of theengine is more reduced when the engine output reduction requirement islarge than when the engine output reduction requirement is small, anengine speed detecting means which detects the engine speed, an engineoutput reduction limiting means which limits reduction of the engineoutput when the engine speed is lower than a preset engine speed, apreset engine speed changing means which changes the preset engine speedaccording to the engine output reduction requirement so that the presetengine speed is higher when the engine output reduction requirement islarge than when the engine output reduction requirement is small.